Nitride compound semiconductor light emitting device and method for producing the same

ABSTRACT

A nitride compound semiconductor light emitting device includes: a GaN substrate having a crystal orientation which is tilted away from a &lt;0001&gt; direction by an angle which is equal to or greater than about 0.05° and which is equal to or less than about 2°, and a semiconductor multilayer structure formed on the GaN substrate, wherein the semiconductor multilayer structure includes: an acceptor doping layer containing a nitride compound semiconductor; and an active layer including a light emitting region.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nitride compound semiconductor lightemitting device formed on a GaN substrate, and a method for producingthe same.

2. Description of the Related Art

Conventionally, nitride compound semiconductor light emitting deviceshave been studied or utilized as light emitting devices or high powerdevices. In the case of light emitting devices, for example, lightemitting devices covering a wide range of colors from blue to orange canbe technically realized by using nitride compound semiconductors ofvarious compositions. By taking advantage of such properties of nitridecompound semiconductors, blue and green light emitting diodes (LEDs)have been realized in recent years. As for laser devices, blue-violetsemiconductor laser devices are under development.

By using a nitride compound semiconductor, a light emitting device istypically produced as follows. A current injection layer having n-typeproperties is formed on a substrate, e.g., a mirror-polished sapphire(0001) substrate, upon which a nitride compound semiconductor is to beepitaxially grown. An active layer and a current injection layercontaining an acceptor impurity are further formed thereupon. It isknown that the use of a quantum well layer having a thickness of about10 nm or less for an active layer results in a high emission intensity.The emission wavelength can be varied by adjusting the In (indium)component ratio in an InGaN active layer, for example. After the entirelight emitting device structure has been formed, the device is subjectedto a heat treatment in an N₂ gas, whereby the acceptor is activated, soas to impart p-type properties thereto. Thus, an LED or a laser deviceis completed.

In general, by doping an n-type nitride compound semiconductor crystalwith Si using a SiH₄ gas during a crystal growth process, for example,an electron density of 10¹⁸ cm⁻³ or more can be easily obtained. On theother hand, in order to obtain a hole density on the order of 10¹⁸ cm⁻³with a p-type nitride compound semiconductor crystal, it is necessary todope the p-type nitride compound semiconductor crystal with Mg usingCP₂Mg (Bis(cyclopentadienyl)Magnesium) or EtCP₂Mg(Bis(Ethylcyclopentadienyl) Magnesium) during a crystal growth process,and after the entire light emitting device structure including an activelayer has been formed, subject the device to a heat treatment in aninert gas such as N₂. A p-crystal having a high hole density “as grown”has not hitherto been obtained.

As used herein, a “high hole density” means a density of 10¹⁷ cm⁻³ ormore. The expression “as grown” is used to describe a device which,after crystal growth has taken place, has not been subjected to a heattreatment, electron beam irradiation, etc. An “acceptor doping layer”means a layer which has been doped with an acceptor impurity, e.g., Mg.

The reason why an acceptor doping layer does not exhibit a p-typeconductivity “as grown” under the conventional methods is that Mg atomswhich are taken into the mother crystal have been inactivated byhydrogen. Specifically, a nitride compound semiconductor crystal whichhas been formed on a conventional sapphire substrate has a highconcentration of defects and/or nitrogen vacancies due to a latticemismatch as high as 13% with the sapphire substrate. Therefore, Mg atomscannot be taken into the crystal by themselves, but rather are entrappedin an inactive state, i.e., Mg—H. Accordingly, in order to sever theMg—H bonds so as to obtain active Mg atoms, it is necessary to applythermal energy at a temperature on the order of several hundred ° C. inan inert gas atmosphere free of hydrogen after the light emitting devicestructure has been formed.

However, even after a heat treatment, which damages a thermally unstableactive layer containing In, the resultant hole density would be betweena latter half of the 10¹⁷ cm⁻³ order to the 10¹⁸ cm⁻³ order. To reducethe operation voltage in a light emitting device, it is necessary toreduce a contact resistance when a p-type is electrode has been formed.Therefore, there is a desire for achieving an increased hole density ina p-type layer. In particular, a device which operates with a highcurrent density, e.g., a laser device, is likely to be heated due to ahigh contact resistance, so that degradation may begin from an interfacebetween the electrode and the p-type layer, leading to electrodedestruction. In addition, excessive heating might cause deteriorationassociated with the mobility of or increase in dislocations within thelight emitting device, resulting in a decrease in the emission intensityor fluctuation in the emission wavelength. Thus, the low hole densitylevel in a p-type layer presently achievable under the conventionaltechnique is detrimental to the emission characteristics and/orlongevity of light emitting devices.

Moreover, a light emitting device formed on a conventional sapphiresubstrate not only suffers from the inactivation of Mg, but alsoreceives unfavorable influences on an InGaN multiple quantum well activelayer. As mentioned above, a light emitting device which includes anInGaN quantum well active layer formed on a sapphire substrate has asubstantially incommensurate lattice constant with that of the sapphiresubstrate, and hence has a high concentration of nitrogen vacanciesand/or threading dislocations, i.e., dislocations penetrating the devicefrom the substrate interface to the device surface through the quantumwell structure. In particular, a current which flows through a threadingdislocation is a component which does not contribute to emission, andtherefore increases the driving current density in the light emittingdevice, inducing heating within the light emitting device. Moreover,since a nitride compound semiconductor containing In is very unstable interms of chemical-thermal equilibrium during a crystal growth process, ahigh concentration of dislocations are present. In the presence of sucha high level of undulation in the underlying layer, each layer in themultiple quantum well structure formed thereon will have a non-uniformthickness.

As a method for solving the problems of dislocations and nitrogenvacancies, Japanese Laid-Open Patent Publication No. 9-23026 discloses atechnique of performing a two-part growth involving a buffer layer,where an angle between a sapphire substrate and the (0001) plane ismaintained equal to or less than 5°, thereby reducing dislocations andimproving emission characteristics. There is also disclosed a techniqueof, after growing a single quantum well active layer of InGaN,interrupting the growth or observing a watt period for 60 minutes orless to obtain a light emitting device having a uniform emission stateand high yield. Japanese Laid-Open Patent Publication No. 10-126006discloses that a quantum well laser device having a low thresholdcurrent density can be formed by forming a well layer to become anactive layer in a three-well quantum well structure, observing a waitperiod for 2 to 10 seconds, and then forming semiconductor layers.

However, in all of the aforementioned conventional techniques, a heattreatment for imparting p-type properties is required after forming alight emitting device structure. Due to an insufficient carrier density,a sufficiently low p-type contact resistance has not been realized. Inaddition, the problems associated with a heat treatment for impartingp-type properties, e.g., a damaged active layer, non-uniform compositionof In-containing layers, non-uniform layer thicknesses, deterioration incrystal quality, etc., have not been solved. Therefore, it is difficultwith conventional techniques to produce a high-efficiency LED or alow-threshold semiconductor laser device which requires a reducedoperation voltage and/or current. Thus, there to a need for a techniqueof producing light emitting devices having improved characteristics.

SUMMARY OF THE INVENTION

A nitride compound semiconductor light emitting device according to thepresent invention includes: a GaN substrate having a crystal orientationwhich is tilted away from a <0001> direction by an angle which is equalto or greater than about 0.05° and which is equal to or less than about2°, and a semiconductor multilayer structure formed on the GaNsubstrate, wherein the semiconductor multilayer structure includes: anacceptor doping layer containing a nitride compound semiconductor; andan active layer including a light emitting region.

In one embodiment of the invention, the acceptor doping layer iscomposed essentially of Ga_(x)In_(y)Al_(1−(x+y))N (where 0≦x≦1; 0≦y≦1;and 0≦x+y≦1).

In another embodiment of the invention, the GaN substrate has a crystalorientation which is tilted away from a <0001> direction in a <11-20> or<1-100> direction.

In still another embodiment of the invention, the acceptor doping layerexhibits a p-type conductivity as grown.

In still another embodiment of the invention, the GaN substrate and theactive layer are formed so as to be apart from each other by a distancewhich is equal to or greater than about 1 μm.

In still another embodiment of the invention, the active layer has aquantum well structure, and the active layer has an averaged surfaceroughness which is equal to or less than a thickness of a well layer inthe quantum well structure.

In still another embodiment of the invention, the active layer includesat least one well layer and at least one barrier layer.

A method for, producing the nitride compound semiconductor lightemitting device according to the present invention includes: after atleast one of the at least one well layer and the at least one barrierlayer has been formed, observing a wait period during which no otherlayers are formed, the wait period having a predetermined length.

In one embodiment of the invention, the predetermined length of the waitperiod is equal to or greater than about 1 second and is equal to orless than about 60 minutes.

In another embodiment of the invention, the method further includes:supplying a carrier gas into a chamber, in which the GaN substrate isplaced, during the wait period after at least one of the at least onewell layer and the at least one barrier layer has been formed, thecarrier gas containing nitrogen as a main component.

In still another embodiment of the invention, the method furtherincludes: supplying a carrier gas and a group V gas Into a chamber, Inwhich the GaN substrate is placed, during the wait period after at leastone of the at least one well layer and the at least one barrier layerhas been formed, the carrier gas containing nitrogen as a maincomponent.

Thus, the invention described herein makes possible the advantages ofproviding a high-intensity nitride compound semiconductor light emittingdevice which emits light with a low operation voltage and/or current.

These and other advantages of the present invention will become apparentto those skilled in the art upon reading and understanding the followingdetailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic cross-sectional view illustrating a semiconductorlight emitting device produced by using a nitride compound semiconductoraccording to the present invention.

FIG. 1B is a schematic cross-sectional view illustrating the structureof an active layer of a light emitting device produced according to thepresent invention.

FIG. 1C is a flow chart illustrating steps for producing an active layerof a light emitting device according to one embodiment of the invention.

FIG. 1D is a flow chart illustrating steps for producing an active layerof a light emitting device according to another embodiment of theinvention.

FIG. 2 is a schematic cross-sectional view illustrating a manner inwhich growth cores may form in a case of a substrate surface having arelatively large tilt angle.

FIG. 3 is a schematic cross-sectional view illustrating a manner inwhich growth cores may form in a case where a tilt angle from the <0001>direction is defined according to the present invention, whereby surfacesteps are optimized.

FIG. 4 is a schematic block diagram illustrating a crystal growthapparatus which may be used in the present invention.

FIG. 5 is a graph illustrating a schedule of growth temperatures in thevicinity of an active layer and flow rates of respective materialsaccording to the present invention;

FIG. 6 is a graph illustrating a relationship between the tilt angle ofa substrate surface and hole density according to the present invention.

FIG. 7 is a graph illustrating a relationship between the tilt angle ofa substrate surface, threading dislocation density, and averaged surfaceroughness according to the present invention.

FIG. 8 shows a relationship between the tilt angle of substrate surfaceand emission intensity of a semiconductor light emitting deviceaccording to the present invention.

FIG. 9 is a graph illustrating a relationship between the tilt angle ofsubstrate surface and emission intensity of a semiconductor lightemitting device according to the present invention, where the growthtemperature for an active layer is varied.

FIG. 10 is a graph illustrating a relationship between the totalthickness of underlying n-GaN layers and averaged surface roughness of auppermost growth surface according to the present invention.

FIG. 11 is a graph illustrating a relationship between the totalthickness of underlying layers and emission intensity of a semiconductorlight emitting device according to the present invention.

FIG. 12 is a graph illustrating a relationship between wait periodobserved after growing each barrier layer and emission intensity of asemiconductor light emitting device according to the present invention.

FIG. 13 is a graph illustrating a relationship between wait periodobserved after growing each well layer and emission intensity of asemiconductor light emitting device according to the present invention.

FIG. 14 is a graph illustrating a relationship between nitrogen partialpressure, emission intensity, and emission wavelength of a semiconductorlight emitting device according to the present invention.

FIG. 15 is a graph illustrating a relationship between the flow rate ofNH₃ supplied during a wait period and emission intensity of asemiconductor light emitting device according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 presents a schematic microscopic observation of surfaces ofsemiconductor layers grown on a substrate 201 not satisfying a crystalorientation range constraint defined by the present invention. As seenfrom FIG. 2, there is a non-uniform distribution of steps 202 over thesubstrate 201, the steps 202 having a height on the order of a fewatomic layers. As a result, a three-dimensional growth mode (i.e. localprogress of growth) is predominant because non-uniform growth cores 203hinder an organized step flow growth. Thus, a high concentration ofthreading dislocations and/or nitrogen vacancies may occur with thegrowth of semiconductor layers in a thickness direction. Penetrationdislocations and/or nitrogen vacancies may result in entrapment of Mg—H.Moreover, a crystal having a high concentration of threadingdislocations and/or nitrogen vacancies has a substantially undulateduppermost surface. Such a surface does not provide an appropriateunderlying layer for forming a quantum well structure thereon. Inparticular, it may affect the crystallinity and composition uniformityof an light emitting layer containing In.

On the other hand, FIG. 3 presents a schematic microscopic observationof surfaces of semiconductor layers grown on a substrate 301 whosecrystal orientation is tilted away from a <0001> direction by a slightangle in the range from about 0.05° to about 2° in a <11-20> or <1-100>direction. As shown in FIG. 3, steps 302 on the substrate 301 aredistributed in an optimum, uniform manner. As a result, atwo-dimensional growth mode occurs in which material species which havearrived at the substrate surface in a vapor phase repeat migration andrevaporization, forming a uniform distribution of growth cores 303 overthe entire substrate surface, whereby a layer-by-layer planar growthoccurs. Thus, the generation of threading dislocations and/or nitrogenvacancies is effectively reduced. Especially in the case of growing ap-type crystal, Mg atoms can be taken into is the crystal by themselves,so that a p-type crystal having a high hole density can be obtainedwithout requiring a heat treatment in an inert gas. Furthermore, as acontact layer, a cladding layer, an optical guide layer, and the likeare laminated to a total thickness of about 1 μm or more in atwo-dimensional growth mode, the two-dimensional growth progresses basedonly on the ordered lattice array information inherent in the substratesurface. During the growth process, the steps from which thetwo-dimensional growth began will gradually disappear, ultimatelyattaining a very flat uppermost surface. By forming a multiple quantumwell active layer on such a highly flat surface, each layer in themultiple quantum well structure will have a uniform thickness, wherebyemission characteristics are improved. Due to such effects, the p-typecontact resistance can be reduced without particularly performing ap-type properties impartment process, thereby enabling efficient currentinfection. Thus, a long-life light emitting device which experiencesminimum heat generation during use and which has a highly flat surfacecan be provided.

According to the present invention, a light emitting device including anactive layer which is composed of a plurality of layers of In-containingnitride compound semiconductors is produced, using a GaN substrate whosecrystal orientation is tilted very slightly from the <0001> direction,in such a manner that underlying layers below the active layer have atotal thickness of about 1 μm or more. As a result, a growth mode isrealized in which threading dislocations and/or nitrogen vacancies arereduced, and Mg atoms are taken into the mother crystal by themselves.Thus, it is possible to obtain a p-type crystal which is activated to ahigh density, without performing a heat treatment process after thecompletion of the device structure which can damage the active layer. Asa result, a low resistance p-type contact can be obtained, therebyimproving the longevity of the resultant device. By employing the methodaccording to the present invention, threading dislocations are reduced,and hence current paths which do not contribute to emission are reduced.In addition, the flatness of the active layer and the underlying layersis improved. Thus, the thickness of each layer In the InGaN multiplequantum well layers is uniformed, thereby providing improved emissioncharacteristics.

According to another embodiment of the present invention, in addition tousing the aforementioned slightly-tilted GaN substrate, a wait period ofa predetermined length is observed after a barrier layer and/or a welllayer in a multiple quantum well active layer have been formed. As aresult, In atoms which may be taken into the solid phase via a smallnumber of sparsely present dislocations are prevented from beingconcentrated, thereby obtaining a uniform composition. This effect isenhanced by the reduction of threading dislocations as realized by theuse of a slightly-tilted GaN substrate and forming underlying layers toa total thickness of about 1 μm or more on the slightly-tilted GaNsubstrate. In contrast, a high concentration of threading dislocationsare present in the crystal grown on a conventional substrate, and Inatoms do not diffuse throughout the layers, but rather concentratearound the dislocations because of a small averaged distance betweendislocations. The use of a slightly-tilted GaN substrate as definedabove alone is effective for reducing the concentration of threadingdislocations, but when combined with a wait period as defined above,also eliminated In concentration around dislocations.

As methods for growing a nitride compound semiconductor, metal organicchemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), andhalide vapor phase epitaxy (HVPE) are in use. Among others, MOCVDtechniques are generally used from the perspective of crystallinity andproductivity. Hereinafter, examples of semiconductor light emittingdevices according to the present invention will be described.

EXAMPLE 1

FIG. 1A is a schematic cross-sectional view illustrating a lightemitting device 1000 according to the present invention. First, a GaNsubstrate 101 whose crystal orientation is tilted away from the <0001>direction by an angle in the range from about 0.05° to about 2° in the<11-20> or <1-100> direction is prepared. In the present specification,a substrate as defined above will be referred to as a “slightly-tiltedsubstrate”.

Upon the slightly-tilted GaN substrate 101, a GN buffer layer 102, ann-GaN layer 103, an n-AlGaN light confining layer 104 for confininglight emitted from an active layer, and a GaN lower optical guide layer105 are laminated in this order to a total thickness of about 1 μm ormore. The GaN buffer layer 102 is optional and can be omitted. The loweroptical guide layer 105 may be doped so as to have n-type properties.Further layers that may generally be used for a light emitting device,e.g., a crack prevention layer and/or a carrier barrier layer, may beadded to the semiconductor light emitting device 1000. Furthermore, theAl component ratio and the electron density in the n-AlGaN lightconfinement layer 104 can be appropriately selected in accordance withdesired device characteristics.

Upon these layers, an active layer 106 as a light emitting region isformed, the active layer 106 including a lamination of InGaN barrierlayers 120 (FIG. 1B) and InGaN well layers 121 (FIG. 1B). FIG. 1B showsthe internal structure of the active layer 106 including alternatelayers of the barrier layers 120 and the well layers 121. The Incomponent ratio in the InGaN barrier layers 120 and the InGaN welllayers 121 in the active layer 106, and the number of repetitive unitseach including one InGaN barrier layer and one InGaN well layer 121 canbe arbitrarily selected in accordance with the desired emissionwavelength. An AlGaN layer 107 for preventing evaporation of InGaNand/or diffusion of an acceptor impurity from p-type layers is formedimmediately above the active layer 106. The thickness and the Alcomponent ratio of the AlGaN layer 107 can be arbitrary selected. Insome embodiments, the AlGaN layer 107 may be omitted.

In contact with the AlGaN layer 107, a GaN optical guide layer 108, anda p-AlGaN light confinement layer 109 are laminated in this order. Thecomponent ratios, thickness, and hole density levels of these layers canbe arbitrarily selected, as is also the case for the GaN lower opticalguide layer 105 and the n-AlGaN light confinement layer 104. As anuppermost layer that is In contact with the p-AlGaN layer 109, a p-GaNcontact layer 110 is provided. On the p-GaN layer 110, a stripe-shapedp-electrode 112 a is formed with an insulation layer 111 interposedtherebetween. The insulation layer 111 is provided for the purpose ofcurrent confinement. An n-electrode 112 b is formed on a bottom face ofthe slightly-tilted GaN substrate 101.

Although the slightly-tilted GaN substrate 101 used An the multilayerstructure illustrated in FIG. 1A is of an n-type, it is also applicableto employ a slightly-tilted p-GaN substrate. In such a case, therespective layers in the multilayer structure shown in FIG. 1A havereverse conductivity types (i.e., n-type layers for p-type layers, andvice versa), and the n-electrode 112 b is formed on the uppermostsurface of the light emitting device, whereas the p-electrode 112 aisformed on the bottom face of the slightly-tilted p-GaN substrate 101.

FIG. 4 is a schematic block diagram illustrating an MOCVD apparatus 2000used for the production of the light emitting device according to thepresent example. In FIG. 4, a GaN substrate 401 is a (0001) plane GaNsubstrate whose crystal orientation is tilted away from the <0001>direction by a slight angle in the range from about 0.05° to about 2° inthe <11-20> or <1-100> direction. The GaN substrate 401 is disposed on asusceptor 402 made of carbon. In the susceptor 402 is provided aresistance-heating type heater made also of carbon (not shown). Thesubstrate temperature can be monitored by means of a thermocouple andcontrolled. A water-cooled reaction tube 403 has a double-tube structureof quartz. Ammonia 406 is used as a group V material, andtrimethylgallium (hereinafter “TMG”) 407 a, trimethylaluminum(hereinafter “TMA”) 407 b, and trimethylindium (hereinafter “TMI”) 407 care used as group III materials by being bubbled with nitrogen orhydrogen gas. SiH₄ 409 is used as an n-type donor material for doping.Bis(cyclopentadienyl) magnesium (hereinafter “CP₂Mg”) 407 d is used as ap-type acceptor material for doping via a material inlet 404. Therespective materials are introduced into the reaction tube 403, withtheir amounts being controlled by means of a respective mass flowcontroller 408, and expelled from an discharge gas outlet 405.

Next, an exemplary crystal growth procedure for forming thesemiconductor light emitting device 1000 as a nitride compoundsemiconductor laser/LED will be described with reference to FIG. 1A.

First, the substrate 101 (corresponding to the GaN substrate 401) iswashed and thereafter placed in the crystal growth apparatus(corresponding to the MOCVD apparatus 2000). The substrate 101 issubjected to a heat treatment in an NH₃ atmosphere for about 10 minutesat about 1100° C., and thereafter cooled to a temperature in the rangefrom about 500° C. to about 600° C. Once a stable temperature isachieved, the carrier gas is changed to nitrogen.

The nitrogen gas is supplied at a total flow rate of about 10 l/min, andammonia is supplied at a flow rate of about 3 l/min. Several secondslater, TMG is supplied at a flow rate of about 20 μmol/min. Thus, theGaN buffer layer 102 is grown for 1 minute at a relatively lowtemperature so as to have a thickness of about 20 nm. After stopping thesupply of TMG and elevating the temperature to about 1050° C., TMG andan SiH₄ gas are supplied at flow rates of about 50 μmol/min and about 10nmol/min, respectively. Thus, the n-GaN layer 103 is grown so as to havea thickness of about 4 μm.

Next, TMA is supplied at a flow rate of about 10 μmol/min, therebygrowing the n-Al_(0.15)Ga_(0.85)N light confinement layer 104 so an tohave a thickness of about 0.5 μm. Note that the light confinement layer104 is not required when producing an LED. Next, the supply of TMA isstopped, and the GaN optical guide layer 105 is grown so as to have athickness of about 0.1 μm. The optical guide layer 105 is not requiredwhen producing an LED.

Thereafter, the supply of SiH₄ and TMG is stopped, and the substratetemperature is lowered to a temperature in the range from about 850° C.to about 700° C. The substrate temperature at this stage serves as aparameter which determines the emission wavelength of the resultantlight emitting device. The lower the temperature, the longer theemission wavelength. The aforementioned substrate temperature range ofabout 850° C. to about 700° C. is for producing violet to green lightemitting devices; a different temperature range can be used to produce alight emitting device which is outside the violet to green spectrum.

Once a stable temperature is achieved, TMG and TMI are suppliedrespectively at a flow rate of about 10 μmol/min, thereby forming anIn_(0.05)Ga_(0.95)N barrier layer 120 (FIG. 1B; having a thickness ofabout 5 nm) of the active layer 106. During the growth of the activelayer, SiH₄ may be supplied at a flow rate of about 10 nmol/min. Afterthe growth of the barrier layer 120 is complete, the supply of TMG andTMI is temporarily stopped, and a wait period (from about 1 second toabout 60 minutes) is observed while supplying a carrier gas and an NH₃gas. Thereafter, TMG and TMI are supplied at flow rates of about 10μmol/min and about 50 nmol/min, respectively. Thus, anIn_(0.05)Ga_(0.95)N well layer 121 (FIG. 1B; having a thickness of about3 nm) of the active layer 106. After the growth of the well layer 121,the supply of TMG and TMI is again stopped, and a wait period (fromabout 1 second to about 60 minutes) is observed while supplying acarrier gas and an NH₃ gas.

The growth of well layers 121 and barrier layers 120 of the active layer106 is repeated. After a multiple quantum well structure having adesired number layers has been formed, a final barrier layer 120 isgrown, thereby completing the growth of the active layer 106. It hasbeen found that a light emitting device having 2 to 5 well layers 121generally has an optimum emission efficiency.

After the growth of the active layer 106, TMG (about 10 μmol/min), TMA(about 5 μmol/min), and CP₂Mg are supplied, thereby growing the AlGaNlayer 107 (having a thickness of about 30 nm) in order to preventsublimation of the InGaN layer. Thereafter, the supply of TMG, TMA, andCP₂Mg is stopped, and the substrate temperature is again elevated toabout 1050° C. After the temperature elevation, TMG (about 50 μmol/min)and CP₂Mg are supplied, thereby growing the GaN optical guide layer 108so as to have a thickness of about 0.1 μm. The optical guide layer 108is not required when producing an LED.

Next, TMA is supplied at a flow rate of about 10 μmol/min, therebygrowing the p-Al_(0.15)Ga_(0.85)N light confinement layer 109 so as tohave a thickness of about 0.5 μm. The light confinement layer 109 is notrequired when producing an LED. After completing the growth, the supplyof TMA is stopped, the p-GaN layer 110 is grown so as to have athickness of about 0.5 μm. After completing the growth, the supply ofTMG and CP₂MG is stopped, and the substrate heating is stopped.

FIG. 5 is a graph illustrating a schedule of growth temperatures in thevicinity of the active layer and the flow rates of respective materialsaccording to the present example of the invention. Illustrated in FIG. 5are: wait periods 501, barrier layer growth periods 502, well layergrowth periods 503, an n-GaN layer growth period 504, a p-GaN layergrowth period 505, and an AlGaN sublimation prevention layer growthperiod 506.

Once the substrate temperature becomes equal to about room temperature,the substrate 101 is taken out from the crystal growth apparatus. Aportion of the n-GaN layer 103 is exposed through reactive ion etching,and the insulation layer 111 of a desired configuration, the p-electrode112 a, and the n-electrode 112 b are formed by a vapor depositionmethod. The substrate is cleaved to create end faces through which lightcan be emitted.

In the case where the semiconductor light emitting device 1000 is formedas an LED, it is unnecessary to form end faces by cleaving thesemiconductor multilayer structure. Rather, light is emitted through thep-electrode 112 a and/or the n-electrode 112 b.

Although the present example illustrates the use of the GaN buffer layer102 as a low temperature buffer layer, this layer may be omitted.Alternatively, Al_(x)Ga_(1−x)N (0≦x≦1) may be used for this layer,without substantially affecting the production of the semiconductorlight emitting device 1000. The heat treatment in an NH₃ atmosphere mayalso be omitted. It is also possible to apply an elevated temperature inan atmosphere of a carrier gas (whose main component is an inert gas)and NH₃, and start the growth of the underlying GaN layer 102concurrently with the introduction of TMG and/or SiH₄.

In the case of using a GaN substrate 101, it is unnecessary to perform aheat treatment in a hydrogen atmosphere and a growth process for abuffer layer 102 at a low temperature. It is also possible to apply anelevated temperature in an atmosphere of a carrier gas (whose maincomponent is an inert gas) and NH₃, and start the growth of theunderlying GaN layer 103 concurrently with the introduction of TMGand/or SiH₄.

The semiconductor light emitting device 1000 thus produced operates witha reduced operation voltage and/or current as compared with thoserequired for conventional semiconductor light emitting devices, and yethas a greater emission intensity than that provided by conventionalsemiconductor light emitting devices. In the case where thesemiconductor light emitting device 1000 is implemented as a laserdevice, unwanted heating is minimized owing to the low contactresistance in the p-type layers, and deterioration occurs only veryslowly, if at all. Thus, a long-life laser device can be realized. Theseadvantages according to the present invention are obtained for thefollowing reasons.

According to the present invention, a light emitting region is formed ona slightly-tilted GaN substrate 101, so that an organized step flowgrowth is realized, substantially reducing threading dislocations and/ornitrogen vacancies within the crystal. As a result, Mg, as a p-typedopant, can be easily taken into the crystal by itself, i.e., withoutbeing bound to hydrogen atoms. Thus, the acceptor doping layer isimparted with a low-resistance p-type conductivity without requiring anyparticular post-processing after growth, so that there is no need toperform a heat treatment which would damage the active layer 106.

Moreover, by forming the underlying layers between the substrate 101 andthe active layer 106 so as to have a total thickness of about 1 μm ormore, the steps on the uppermost surface of the substrate 101 aresufficiently flattened, so that it is ensured that the active layer 106attains a uniform structure in which the fluctuation in the thicknessesof the barrier layers 120 and the well layers 121 is minimized.

Furthermore, reduced threading dislocations and/or nitrogen vacanciesand the wait period which is observed during the growth process togetherallow the active layer 106 to have a uniform In composition, wherebyareas having a high concentration of In atoms, which would be renderedincapable of light emission, can be substantially eliminated. Inparticular, immediately after an In-containing nitride compoundsemiconductor layer has been grown, the crystal is not in a sound statebecause a nitride compound semiconductor containing In, when grown at ahigh temperature, is in a chemically unstable state, and dislocationspenetrating through the film cause In atoms to concentrate. Therefore,by forming nitride compound semiconductor layers so as to have a totalthickness of about 1 μm or more on a slightly-tilted substrate, thedensity of threading dislocations is reduced and the uppermost surfaceis flattened, and with the further application of heat in a nitrogenatmosphere, the In concentration within the In-containing nitridecompound semiconductor layer is substantially eliminated, whereby astable phase is obtained. Thus, the crystal attains a sound state. Inparticular, the crystallinity of the barrier layers 120 adjoining thewall layers 121, which contribute to the light emission is greatlyimproved.

EXAMPLE 2

In Example 2 of the present invention, a semiconductor light emittingdevice 1000 in the form of an LED is produced, using a slightly-tiltedsubstrate 101. In the present example, the relationship between atilting angle of a slightly-tilted substrate and density of threadingdislocations present in the semiconductor light emitting device 1000,surface roughness, and emission intensity with current injection will bediscussed.

By using a GaN substrate 101 having a mirror-polished (0001) plane whosecrystal orientation is actually tilted away from the <0001> direction bya slight angle in the range from about 0.02° to about 5° in the <11-20>or <1-100> direction, a nitride compound semiconductor multilayerstructure is grown in the manner shown in Example 1.

After an n-GaN layer 103 is formed, growth conditions for the activelayer 106 are adjusted so that a constant substrate temperature ismaintained while supplying NH₃. Once a stable substrate temperature isachieved, TMG, TMI, and SiH₄ are supplied, at flow rates of about 10μmol/min, about 10 μmol/min, and 5 nmol/min, respectively, therebyforming an In_(0.05)Ga_(0.95)N barrier layer 120 (FIG. 1B) within anactive layer 106 so as to have a thickness of about 5 nm. Next, TMG,TMI, and SiH₄ are supplied, at flow rates of about 10 μmol/min, about 50μmol/min, and 5 nmol/min, respectively, thereby forming anIn_(0.2)Ga_(0.)N well layer 121 within the active layer 106 so as tohave a thickness of about 3 nm. After growing the well layer 121, theTMG supply is reduced to about 10 μmol/min, and another barrier layer120 within the active layer 106 is grown. After that barrier layer 120has been grown, another well layer 121 is grown, and so forth; thisprocess is repeated until a final barrier layer 120 is grown.

Thereafter, an AlGaN layer 107 for preventing sublimation of the InGaNlayer is grown so as to have a thickness of about 30 nm, following themethod described in Example 1. According to the present example, theactive layer 106 includes three well layers 121. After growing the AlGaNlayer 107, a p-type semiconductor multilayer structure is formed, andelectrodes are formed, in the manner described in Example 1. Thus, thesemiconductor light emitting device 1000 as an LED is completed.

FIG. 6 shows results of a hole density plotting for the Mg doped layersof an actual semiconductor light emitting device 1000 produced inaccordance with the above method, the plotting being obtained through ahole measurement before electrodes are provided on the semiconductorlight emitting device 1000. FIG. 7 shows a relationship between thethreading dislocation density in the semiconductor light emitting device1000 according to the present example as evaluated through across-section TEM observation, and the surface roughness as measuredwith a stepmeter. In FIGS. 6 and 7, “●” plots represent results obtainedwith a substrate tilt in the <1-100> direction from the <0001>direction; and “603 ” plots represent results obtained with a substratetilt in the <11-20> direction from the <0001> direction. In eithercases, when the tilt angle of the substrate is in the range from about0.020° to about 0.045° or in the range from about 2.1° to about 5°, thetilt in the substrate surface causes crystal malformation, and hence ahigh concentration of threading dislocations and substantial surfaceroughness. As a result, the hole density in those cases was too low tobe measured. Moreover, dot-like regions having a diameter of several nmwere observed in the active layer 106 due to In concentration.

On the other hand, when the tilt angle of the substrate is in the rangefrom about 0.05° to about 2°, the threading dislocations were reduced,and the hole density was at least 10¹⁷ cm³¹ ³ or more. Thus, it will beappreciated that a sufficient hole density can be obtained “as grown”according to the present example of the invention. In addition, theaveraged surface roughness of the active layer 106 was reduced to about1.8 nm or less, which is sufficiently smaller than the thickness of eachindividual well layer In the quantum well structure. A cross-section TEMobservation revealed that the surface flatness was already improved atthe time of growing the underlying n-GaN layer 103. Since the reducedthreading dislocation density substantially eliminates the Inconcentration in the active layer 106, substantially non-uniformdot-like regions were scarcely observed. Thus, improved flatness of theunderlying layers improved the fluctuation in the thickness of thequantum well structure active layer 106.

FIG. 8 shows a relationship between the emission intensity and the tiltangle of the substrate tilted in the <11-20> direction or <1-100> fromthe <0001> direction, in a case where a 20 mA current was flowed in thesemiconductor light emitting device 1000 according to the presentexample of the invention. FIG. 9 shows measurement results of theemission intensity when the growth temperature for the active layer 106was varied between about 700° C., about 750° C., and about 800° C. InFIG. 9, “●” plots represent results obtained with a growth temperatureof about 700° C.; “◯” plots represent results obtained with a growthtemperature of about 750° C.; and “Δ” plots represent results obtainedwith a growth temperature of about 800°C. As seen from FIGS. 8 and 9,the emission intensity is enhanced when the tilt angle of the substrateis in the range from about 0.05° to about 2°, although the influence ofthe tilt angle of the substrate on emission intensity has a slightdependence on the growth temperature for the active layer 106. As seenfrom the results shown in FIGS. 7, 8, and 9, there is a clearcorrelation between threading dislocations and emission intensity. Thus,it has been found that the semiconductor light emitting device 1000according to the present invention provides an emission intensity whichis equal to or greater than that provided by a semiconductor lightemitting device produced according to a conventional technique, whilerequiring a smaller driving current. This indicates that the currentpaths not contributing to emission are reduced according to the presentinvention. Although the active layer 106 according to the presentexample is illustrated as including three well layers 121, it has beenfound that similar effects to those provided under the present examplecan be obtained with multiple quantum well structures having two welllayers 121, or any number of well layers between four to ten.

It has also been found that, in the case where the semiconductor lightemitting device 1000 is produced as a laser device using aslightly-tilted GaN substrate 101 having a tilt angle from about 0.05°to about 2°, the threshold current density at which oscillation beginsis decreased with improved emission intensity, and that emissionintensity for the same level of current is improved relative to thatprovided by a semiconductor light emitting device produced according toa conventional technique.

EXAMPLE 3

A semiconductor light emitting device 1000 was produced by a methodsimilar to Example 2 while varying the thickness of the n-GaN layer 103,except that the growth temperature for the active layer 106 was fixed atabout 750° C. FIG. 10 shows results of a surface roughness plotting forthe semiconductor light emitting device 1000 according to the presentexample with respect to the total thickness of the underlying layersbetween the substrate 101 and the active layer 106.

In FIG. 10, “●” plots represent results obtained with a substrate tiltangle of 0.15° in the <1-100> direction from; “◯” plots representresults obtained with a substrate tilt angle of 5° in the <1-100>direction; “♦” plots represent results obtained with a substrate tiltangle of 1.7° in the <11-20> direction; and “⋄” plots represent resultsobtained with a substrate tilt angle of 0.04° in the <11-20> direction.From FIG. 10, it can be seen that, irrespective of the crystalorientation, the flatness of the uppermost surface of the device isimproved with an increased thickness when the tilt angle of thesubstrate is in the range from about 0.15° to about 1.7°. Moreover, inthe case where the semiconductor light emitting device 1000 is producedso that the underlying layers between the substrate 101 and the activelayer 106 have a total thickness of about 1 μm or more, the uppermostsurface has a surface roughness which is smaller than the thickness ofeach individual layer in the quantum well structure. Thus, it has beenindicated that a satisfactory quantum well structure can be obtainedwhen the underlying layers between the substrate 101 and the activelayer 106 have at least a total thickness of about 1 μm in or more.

Although the present example illustrates various thicknesses of the GaNlayer 103, similar results were also obtained with underlying layerscomposed of InGaN or AlGaN. The same tendency was also observed in thecase where the underlying layers are composed of a plurality of InAlGaNlayers of different compositions; i.e., the flatness of the uppermostsurface of the resultant device was improved with a total underlyinglayer thickness of about 1 μm or more, irrespective of the compositionor the number of layers included.

FIG. 11 shows a relationship between the emission intensity and thetotal thickness of the underlying layers between the GaN substrate 101and the active layer 106 with respect to the semiconductor lightemitting device 1000 produced on a mirror-polished slightly-tilted GaNsubstrate 101 while varying the thickness of the n-GaN layer 103, with a20 mA current being applied via the electrodes. In FIG. 11, “●” plotsrepresent results obtained with a substrate tilt angle of 0.150° in the<1-100> direction from; and “◯” plots represent results obtained with asubstrate tilt angle of 0.17° in the <11-20> direction. From FIG. 11, itcan be seen that, in either case, an improved emission intensity isprovided when the underlying layers between the substrate 101 and theactive layer 106 have a total thickness of about 1 μm or more. This ispresumably because the improved flatness of the uppermost surface of thedevice leads to reduced fluctuation in the thickness of the active layer106 and reduced fluctuation in the In component ratio.

Although the active layer 106 according to the present example isillustrated as including three well layers 121, it has been found thatsimilar effects to those provided under the present example can beobtained with multiple quantum well structures having two well layers121, or any number of well layers 121 between four to ten.

It has also been found that, in the case where the semiconductor lightemitting device 1000 is produced as a laser device, using aslightly-tilted GaN substrate 101 having a tilt angle from about 0.05°to about 2° in such a manner that the underlying layers between thesubstrate 101 and the active layer 106 have a total thickness of about 1μm or more, the threshold current density at which oscillation begins isdecreased with improved emission intensity, and that emission intensityfor the same level of current is improved relative to that provided by asemiconductor light emitting device produced according to a conventionaltechnique.

EXAMPLE 4

In Example 4 of the present invention, a semiconductor light emittingdevice 1000 in the form of an LED is produced, using the above-describedgrowth wait period technique on a slightly-tilted substrate 101. In thepresent example, the relationship between the emission intensity when acurrent is injected to the resultant LED and the waiting time observedafter growing each barrier layer 120 in the active layer 106 will bediscussed.

By using a GaN substrate having a mirror-polished (0001) plane whosecrystal orientation is actually tilted away from the <0001> direction by0.15° in the <1-100> direction, a nitride compound semiconductormultilayer structure it grown in the manner shown in Example 1.

Now, steps for producing the active layer 106 according to the presentexample of the invention will be described with reference to a flowchart shown in FIG. 1C. After an n-GaN layer 103 is formed, growthconditions for the active layer 106 are adjusted so that a constantsubstrate temperature is maintained while supplying NH₃. Once a stablesubstrate temperature is achieved, TMG, TMI, and SiH₄ are supplied, atflow rates of about 10 μmol/min. about 10 μmol/min, and 5 nmol/min,respectively, thereby forming an In_(0.05)Ga_(0.95)N barrier layer 120(FIG. 1B) within an active layer 106 so as to have a thickness of about5 nm (step S130). Next, the supply of TMG, TMI, and SiH₄ is stopped, anda predetermined wait period is observed while supplying a carrier gasand an NH₃ gas (step S131). Thereafter, TMG, TMI, and SiH₄ are againsupplied, at flow rates of about 10 μmol/min, about 50 μmol/min, and 5nmol/min, respectively, thereby forming an In_(0.2)Ga_(0.6)N well layer121 within the active layer 106 so as to have a thickness of about 3 nm(step S132). After growing the well layer 121, the TMG supply is reducedto about 10 μmol/min, and another barrier layer 120 within the activelayer 106 is grown. After that barrier layer 120 has been grown, apredetermined Wait period is observed, and then another well layer 121is grown, and so forth; this process is repeated until a final barrierlayer 120 is grown (step S133).

Thereafter, an AlGaN layer 107 for preventing sublimation of the InGaNlayer is grown so as to have a thickness of about 30 nm, following themethod described in Example 1. A wait period may or may not be observedbetween the growth of the final InGaN barrier layer 120 In the activelayer 106 and the growth of the AlGaN layer 107. However, it has beenfound that in the case where the active layer 106 includes two or lesswell layers 121, observing a wait period after the growth of the finalInGaN barrier layer 120 in the active layer 106 makes for a higheremission intensity responsive to a current injection in thesemiconductor light emitting device 1000. According to the presentexample, the active layer 106 includes three well layers 121.

After growing the AlGaN layer 107, a p-type semiconductor multilayerstructure is formed, and electrodes are formed, in the manner describedin Example 1. Thus, the semiconductor light emitting device 1000 as anLED is completed.

FIG. 12 shows a relationship between the emission intensity and thewaiting time observed after forming each barrier layer 120, In a casewhere a 20 mA current was flowed in the semiconductor light emittingdevice 1000 according to the present example of the invention. FIG. 12shows measurement results of the emission intensity when the growthtemperature for the active layer 106 was varied between about 700° C.about 750° C., and about 800° C. In FIG. 12, “●” plots represent resultsobtained with a growth temperature of 700° C.; “◯” plots representresults obtained with a growth temperature of 750° C.; and “Δ” plotsrepresent results obtained with a growth temperature of about 800° C. Adotted line in FIG. 12 represents an emission intensity of 400 a.u.(arbitrary units), which is obtained when a zero waiting time isobserved for each growth temperature. The intensity level denoted by the“●”, “◯”, or “Δ” symbol represents an average emission intensityassociated with each growth temperature.

As seen from FIGS. 8, 9, and 12, the emission intensity is furtherenhanced by applying a growth wait period technique in addition to theuse of a slightly-tilted substrate 101.

As seen from FIG. 12, the emission intensity is enhanced when a waitingtime of 1 second or more is observed, although the influence of thewaiting time on emission intensity has a slight dependence on the growthtemperature for the active layer 106. A relatively long waiting timeprovides a significant improvement on the emission intensity in the casewhere a low growth temperature is used for forming the active layer 106;on the other hand, a relatively short waiting time provides asignificant improvement on the emission intensity in the case where ahigh growth temperature is used for forming the active layer 106.

Specifically, as seen from FIG. 12, in the case where the growthtemperature for the active layer 106 is about 700° C., a waiting time inthe range from about 1 second to about 60 minutes provides a significantimprovement on the emission intensity, and a waiting time in the rangefrom about 1 second to about 10 minutes provides a particularlysignificant improvement. In the case where the growth temperature forthe active layer 106 in about 750° C., a waiting time in the range fromabout 1 second to about 15 minutes provides a significant improvement onthe emission intensity, and a waiting time in the range from about 1second to about 5 minutes provides a particularly significantimprovement. In the case where the growth temperature for the activelayer 106 is about 800° C., a waiting time in the range from about 1second to about 5 minutes provides a significant improvement on theemission intensity, and a waiting time in the range from about 1 secondto about 2 minutes provides a particularly significant improvement.

It should also be noted that the above effect was most prominent whenthe time spent for growing each pair of a barrier layer 120 and a welllayer 121, including the waiting time observed in between, was in therange from about 10 seconds to about 120 minutes.

Although the present example illustrates the use of a slightly-tiltedGaN substrate 101 having a mirror-polished (0001) plane whose crystalorientation is actually tilted away from the <0001> direction by 0.15°in the <1-100> direction, It has been confirmed that a tilt in any otherdirection exhibits similar effects so long as the tilt angle is in therange from about 0.05° to about 2°.

Although the active layer 106 according to the present example isillustrated as including three well layers 121, it has been found thatsimilar effects to those provided under the present example can beobtained with multiple quantum well structures having two well layers121, or any number of well layers 121 between four to ten.

It has also been found that in the case where the semiconductor lightemitting device 1000 is produced as a laser device by a method accordingto the present example, utilizing the above-described growth wait periodtechnique, the threshold current density at which oscillation begins isdecreased with improved emission intensity, and that emission intensityfor the same level of current is improved relative to that provided by asemiconductor light emitting device produced according to a conventionaltechnique.

EXAMPLE 5

In Example 5 of the present invention, a semiconductor light emittingdevice 1000 in the form of an LED is produced, using the above-describedgrowth wait period technique on a GaN substrate having a mirror-polished(0001) plane whose crystal orientation is actually tilted away from the<0001> direction by 0.15° in the <1-100> direction. According to thepresent example, a predetermined wait period is observed after growingeach barrier layer 120 within an active layer 106, and a predeterminedwait period is observed after growing each well layer 121 within anactive layer 106. In the present example, the relationship between theemission intensity when a current is injected to the resultant LED andthe waiting time observed after growing each well layer 121 in theactive layer 106 will be discussed. Thus, according to the presentexample, the step of leaving the substrate 101 for a predeterminedwaiting time (step S131) in the flowchart shown in FIG. 1C is performednot only after the formation of each barrier layer 120 but also afterthe formation of each well layer 121. Respective steps for forming theactive layer 106 according to the present example of the invention willbe described with reference to a flowchart shown in FIG. 1D. The methodfor growing each layer in the semiconductor light emitting device 1000according to the present example is the same as that described inExample 3. Hereinafter, growth conditions for forming the active layer106 will be described.

After an n-GaN layer 103 is formed, growth conditions for the activelayer 106 are adjusted so that a constant substrate temperature ismaintained while supplying NH₃. Once a stable substrate temperature isachieved, TMG, TMI, and SiH₄ are supplied, at flow rates of about 10μmol/min, about 10 μmol/min, and 5 nmol/min, respectively, therebyforming an In_(0.05)Ga_(0.95)N barrier layer 120 within an active layer106 so as to have a thickness of about 5 nm (step S140). Next, thesupply of TMG, TMI, and SiH₄ is stopped, and a predetermined wait periodis observed while supplying a carrier gas and an NH₃ gas (step S141).Thereafter, TMG, TMI, and SiH₄ are again supplied, at flow rates ofabout 10 μmol/min, about 15 μmol/min, and 5 nmol/min, respectively,thereby forming an In_(0.2)Ga_(0.8)N well layer 121 within the activelayer 106 so as to have a thickness of about 5 nm (step S142). Next, thesupply of TMG, TMI, and SiH₄ is stopped, and a predetermined wait periodis observed while supplying a carrier gas and an NH₃ gas (step S143)Thus, after each barrier layer 120 is grown, a wait period is observed,and after each well layer 121 is grown, another wait period is observed,and so forth; this process of alternately forming adjoining layers ofbarrier layers 120 and well layers 121 is repeated until a final barrierlayer 120 is grown (step S144).

Thereafter, an AlGaN layer 107 for preventing sublimation of the InGaNlayer is grown so as to have a thickness of about 30 nm, following themethod described in Example 1. A wait period may or may not be observedbetween the growth of the final InGaN barrier layer 120 in the activelayer 106 and the growth of the AlGaN layer 107. However, it has beenfound that in the case where the active layer 106 includes two or lesswell layers 121, observing a wait period after the growth of the finalInGaN barrier layer 120 in the active layer 106 makes for a higheremission intensity responsive to a current injection in thesemiconductor light emitting device 1000. According to the presentexample, the active layer 106 includes three well layers 121, and thewaiting time observed after growing each barrier layer 120 is about 60seconds.

After growing the AlGaN layer 107, a p-type semiconductor multilayerstructure is formed, and electrodes are farmed, in the manner describedin Example 1. Thus, the semiconductor light emitting device 1000 as anLED is completed.

FIG. 13 shows a relationship between the emission intensity and thewaiting time observed after forming each well layer 121, In a case wherea 20 mA current was flowed in the semiconductor light emitting device1000 according to the present example of the invention. FIG. 13 showsmeasurement results of the emission intensity when the growthtemperature for the active layer 106 was varied between about 700° C.,about 750° C., and about 800° C. In FIG. 13, “●” plots represent resultsobtained with a growth temperature of 700° C.; “◯” plots representresults obtained with a growth temperature of 750° C.; and “Δ” plotsrepresent results obtained with a growth temperature of about 800° C. Adotted line in FIG. 13 represents an emission intensity of 400 a.u.(arbitrary units), which is obtained when a zero waiting time isobserved for each growth temperature. The intensity level denoted by the“●”, “◯”, or “Δ” symbol represents an average emission intensityassociated with each growth temperature.

As seen from FIGS. 8, 9, and 13, the emission intensity is furtherenhanced by applying a growth waiting technique period in addition tothe use of a slightly-tilted substrate 101.

As seen from FIG. 13, the emission intensity is enhanced when a waitingtime of 1 second or more is observed after the formation of each welllayer 121, although the influence of the waiting time on emissionintensity has a slight dependence on the growth temperature for theactive layer 106. A relatively long waiting time provides a significantimprovement on the emission intensity in the case where a low growthtemperature is used for forming the active layer 106; on the other hand,a relatively short waiting time provides a significant improvement onthe emission intensity in the case where a high growth temperature isused for forming the active layer 106.

Specifically, as seen from FIG. 13, in the case where the growthtemperature for the active layer 106 is about 700° C., a waiting time inthe range from about 1 second to about 60 minutes provides a significantimprovement on the emission intensity, and a waiting time in the rangefrom about 1 second to about 10 minutes provides a particularlysignificant improvement. In the case where the growth temperature forthe active layer 106 is about 750° C., a waiting time in the range fromabout 1 second to about 15 minutes provides a significant improvement onthe emission intensity, and a waiting time in the range from about 1second to about 5 minutes provides a particularly significantimprovement. In the case where the growth temperature for the activelayer 106 is about 800° C., a waiting time in the range from about 1second to about 5 minutes provides a significant improvement on theemission intensity, and a waiting time in the range from about 1 secondto about 2 minutes provides a particularly significant improvement.

In the case where a wait period is observed only after the growth ofeach well layer 121, i.e., without observing a wait period after thegrowth of each barrier layer 120, the emission intensity is somewhatimproved, but to a lesser degree than that illustrated in the graph ofFIG. 12; the improvement in emission intensity is threefold at the most.

EXAMPLE 6

In Example 6 of the present invention, a semiconductor light emittingdevice 1000 in the form of an LED is produced, using a GaN substrate 101having a mirror-polished (0001) plane whose crystal orientation isactually tilted away from the <0001> direction by 0.15° in the <1-100>direction, following the method described in Example 2, while varyingthe mixing ratio between a hydrogen gas and a nitrogen gas in a carriergas which is supplied during a wait period after growing a barrier layer120 within the active layer 106. In the present example, therelationship between the emission characteristics of the semiconductorlight emitting device 1000 and the mixing ratio between a hydrogen gasand a nitrogen gas in a carrier gas which is supplied during a waitperiod after growing a barrier layer 120 within the active layer 106will be discussed.

FIG. 14 shows a relationship between the emission intensity and emissionwavelength of the semiconductor light emitting device 1000 and variousmixing ratios between a hydrogen gas and a nitrogen gas in a carrier gaswhich is supplied during a wait period of about 60 seconds after growinga barrier layer 120, where the total supply rate of the carrier gas ismaintained at a constant level and the growth temperature for the activelayer 106 is fixed at about 750° C. In FIG. 14, “●” plots representemission intensity (left vertical axis), and “◯” plots representemission wavelength (right vertical axis).

As shown in FIG. 14, the emission wavelength and the emission intensitytend to decrease as the N₂ component 10 in the carrier gas decreases.The same tendency is also exhibited when the growth temperature for theactive layer 106 is as high as about 800° C. or as low as about 700° C.In the case where a wait period is observed not only after growing eachbarrier layer 120 but also after growing each well layer 121, anincreased N₂ component in the carrier gas leads to increased emissionintensity and greater emission wavelength.

EXAMPLE 7

In Example 7 of the present invention, a semiconductor light emittingdevice 1000 in the form of an LED is produced, using a GaN substrate 101having a mirror-polished (0001) plane whose crystal orientation isactually tilted away from the <0001> direction by 0.15° in the <1-100>direction, following the method described in Example 2, while varyingthe flow rate of an NH₃ gas supplied during a wait period after growinga barrier layer 120 within the active layer 106. In the present example,the relationship between the emission intensity of the semiconductorlight emitting device 1000 and the flow rate of an NH₃ gas suppliedduring a wait period after growing a barrier layer 120 within the activelayer 106 will be discussed.

FIG. 15 shows a relationship between the emission intensity of thesemiconductor light emitting device 1000 and various flow rates of anNH₃ gas which is supplied during various lengths of wait periods aftergrowing a barrier layer 120, where the growth temperature for the activelayer 106 is fixed at about 750° C. In FIG. 15, “●” plots representresults obtained with NH₃ is supplied at a flow rate of about 5 l/min;“◯” plots represent results obtained with NH₃ is supplied at a flow rateof about 3 l/min; and “Δ” plots represent results obtained with NH₃ issupplied at a flow rate of 0 (zero) l/min.

As shown in FIG. 15, it has been confirmed that the emission wavelengthis enhanced even with a zero flow rate of NH₃; however, the supply ofany non-zero amount of NH₃ has a positive enhancing effect on emissionintensity, and also allows for a long wait period to be used, wherebythe entire production process can be facilitated. The same tendency isalso exhibited when the growth temperature for the active layer 106 isas high as about 800° C. or as low as about 700° C. The same tendency isalso exhibited in the case where a wait period is observed not onlyafter growing each barrier layer 120 but also after growing each welllayer 121.

Examples 1 to 7 chiefly illustrate instances in which the presentinvention is applied to an LED. The inventors have confirmed throughexperimentation that the present invention can be applied to a laserdevice in order to effectively reduce a threshold current value thereof.In fact, semiconductor light emitting devices 1000 which were producedin the form of laser devices, by following the above-described methodsfor realizing high emission intensity in semiconductor light emittingdevices 1000 as LEDs, exhibited laser oscillation with a low thresholdcurrent density. Thus, the present invention is effective for any lightemitting devices generally composed of nitride compound semiconductormaterials.

As described above, according to the present invention, a crystal isgrown on a GaN substrate having a slightly-tilted crystal orientation,whereby a nitride compound semiconductor crystal exhibiting a p-typeconductivity which has a hole density of about 10¹⁷ cm⁻³ or above can beobtained “as grown”, without performing a heat treatment or an electronbeam irradiation. As a result, the contact resistance in the nitridecompound semiconductor light emitting device according to the presentinvention can be effectively reduced, thereby providing for improvedlight emitting device characteristics. By ensuring that the underlyinglayers between the surface of the GaN substrate and an active layer havea total thickness of about 1 μm or more, and observing a predeterminedwait period after growing each barrier layer and/or well layer in amultiple quantum well structure, there is provided a high-luminancenitride compound semiconductor light emitting device which can emitlight with a relatively low driving current and/or operation voltage andwhich is unaffected by heating.

Various other modifications will be apparent to and can be readily madeby those skilled in the art without departing from the scope and spiritof this invention. Accordingly, it is not intended that the scope of theclaims appended hereto be limited to the description as set forthherein, but rather that the claims be broadly construed.

1. A nitride compound semiconductor light emitting device comprising: aGaN substrate having a (0001) plane whose crystal orientation is tiltedaway from a <0001> direction in a <11-20> or <1-100> direction by anangle which is equal to or greater than 0.05° and which is less than 2°,and a n-type layer containing a nitride compound semiconductor locatedabove the GaN substrate, and an active layer containing a nitridecompound semiconductor located above the GaN substrate, and an acceptordoping layer containing a nitride compound semiconductor comprisingGa_(x)In_(y)Al_(1−(x+y))N (where 0≦x≦1; 0≦y≦1; and 0≦x+y≦1) locatedabove the GaN substrate, wherein the GaN substrate and the active layerare formed so as to be apart from each other by a distance which isequal to or greater than about 1 μm.
 2. A nitride compound semiconductorlight emitting device according to claim 1, wherein the acceptor dopinglayer exhibits a p-type conductivity as grown.
 3. A nitride compoundsemiconductor light emitting device according to claim 1, wherein theactive layer has a quantum well structure, and the active layer has anaveraged surface roughness which is equal to or less than a thickness ofa well layer in the quantum well structure.
 4. A nitride compoundsemiconductor light emitting device according to claim 1, wherein theactive layer includes at least one well layer and at least one barrierlayer.
 5. A nitride compound semiconductor light emitting deviceaccording to claim 1, wherein said active layer is formed evenly withrespect to a macroscopic view and a microscopic view relating to anorder of thickness of the active layer.
 6. A nitride compoundsemiconductor light emitting device according to claim 1, wherein saidacceptor doping layer is formed evenly with respect to a macroscopicview and a microscopic view relating to an order of thickness of theactive layer.
 7. A nitride compound semiconductor light emitting devicecomprising: a GaN substrate having a (0001) plane whose crystalorientation is tilted away from a <0001> direction in a <11-20> or<1-100> direction by an angle which is equal to or greater than 0.05°and which is less than 2°, and a n-type layer containing a nitridecompound semiconductor located above the GaN substrate, and an activelayer containing a nitride compound semiconductor located above the GaNsubstrate, and an acceptor doping layer containing a nitride compoundsemiconductor comprising Ga_(x)In_(y)Al_(1−(x+y))N (where 0≦x≦1; 0≦y≦1;and 0≦x+y≦1) located above the GaN substrate, wherein the acceptordoping layer exhibits a p-type conductivity as grown.
 8. A nitridecompound semiconductor light emitting device according to claim 7,wherein the GaN substrate and the active layer are formed so as to beapart from each other by a distance which is equal to or greater thanabout 1 μm.
 9. A nitride compound semiconductor light emitting deviceaccording to claim 8, wherein the active layer has a quantum wellstructure, and the active layer has an averaged surface roughness whichis equal to or less than a thickness of a well layer in the quantum wellstructure.
 10. A nitride compound semiconductor light emitting deviceaccording to claim 7, wherein the acceptor doping layer has a holedensity of 10¹⁷ cm⁻³ or more.